A method and apparatus of this type has already been proposed in U.S. Pat. No. 4,414,833, which method is applicable more generally to any elongate metal element of constant cross section.
However, it has been observed that when implementing such a method on thick tubes, a large temperature gradient appears between the outside surface of the heated zone and its inside surface. In the usual case where the heating collar is an inductor element generating eddy currents in the tube, only the first 15 to 20 mm of the tube thickness from its outside surface are heated by the eddy currents, with the rest of the thickness being heated only by conduction, and therefore remaining at a lower temperature. This large temperature gradient gives rise to a relatively high average plastic stress coefficient. If it is desired to avoid the appearance of faults on the inside surface of the tube, this means that the tube to be bent must be advanced rather slowly, and consequently that the hourly bent tube production rate is less than that which could be achieved using tubes which are sufficiently thin for the temperature of the heated zone to remain substantially constant throughout the entire tube thickness. This difficulty is particularly severe, for example, when using steel tubes which are lightly alloyed with 2 to 2.5% by weight chromium and 0.9 to 1.2% by weight molybdenum using the alloy known as 10 CD 9.10 (DIN designation 10CrMo9.10).
Preferred implementations of the present invention provide a method and an apparatus for bending a thick metal tube while reducing the temperature gradient between the inside and outside surfaces of the heated zone to a relatively small value, thereby ensuring that good metallurgical behavior of the tube to be bent is obtained, and consequently reducing the average plasticity stress coefficient in said zone, thereby enabling the hourly bent tube production rate to be increased.
Further, the detection of the temperature of the heated zone and/or the size of the gap between the heating collar and the periphery of the pipe zone, and the subsequent modification of the heat applied to said zone responsive to the noted detections are effected with respect to bending the thick alloy tubes of this invention in the same manner as described in the disclosure of our prior U.S. Pat. No. 4,414,833 which is incorporated by reference herein.